Remove try/finally from GPU Montgomery executor

PerfectNumbers.Core/MersenneDivisorCycles.cs

@@ -206,9 +206,6 @@ public class MersenneDivisorCycles
             }
         }
 
-        // TODO: Replace this naive doubling fallback with the unrolled-hex generator from
-        // MersenneDivisorCycleLengthGpuBenchmarks so large divisors avoid the millions of
-        // iterations measured in the scalar loop.
         // TODO: Route this miss to an ephemeral single-cycle computation on the device selected by the current
         // configuration (and skip persisting the result) when the divisor falls outside the in-memory snapshot
         // so we respect the large-p memory limits and avoid extra cache locks.
@@ -987,25 +984,11 @@ public class MersenneDivisorCycles
 
     private static ulong CalculateCycleLengthFallback(ulong divisor)
     {
-        // Otherwise, find order of 2 mod divisor
-        // TODO: Switch this scalar fallback to the unrolled-hex stepping sequence once
-        // the generator is shared with CPU callers; the benchmark shows the unrolled
-        // variant winning decisively for divisors >= 131,071.
+        // Otherwise, find order of 2 mod divisor using the shared unrolled-hex stepping sequence.
         // TODO: Expose a GPU-first branch here so high divisors leverage the ProcessEightBitWindows
         // kernel measured fastest in CycleLengthGpuVsCpuBenchmarks, returning the result without
         // storing it in the shared cache.
-        ulong order = 1UL;
-        ulong pow = 2UL;
-        while (pow != 1UL)
-        {
-            pow <<= 1;
-            if (pow >= divisor)
-                pow -= divisor;
-
-            order++;
-        }
-
-        return order;
+        return divisor.CalculateMersenneDivisorCycleLengthUnrolledHexCpu();
     }
 
     [MethodImpl(MethodImplOptions.AggressiveInlining)]